Abstract
The thin walled box beam is the most economical form of construction for medium and long span bridge structures, primarily because of its efficiency in carrying eccentric loads. This together with a number of other advantages, has led to it being adopted for almost all new major bridges. The research described in this thesis, is mainly concerned with the behaviour of single cell rectangular box beams, curved in plan. The behaviour of straight box beams under twisting loads has also been investigated in the first two chapters as an introduction to the main part of this work. Longitudinal restraint against warping displacements and distortion of the cross sections usually occur in actual structures, invalidating Saint Venant's theory of torsion. The resistance to distortion could be provided by the frame action of the walls (continuous diaphragm medium) or by concentrated diaphragms. New theories have been developed in order to take account of these conditions and mention should be made of the early work of Vlasov, Ebner and more recently Argyris, in the analysis of straight box beams. Further developments of Vlasov and Ebner's theories for the analysis of single symmetrical rectangular box beams have been made, among them the methods of analysis developed primarily for bridge structures by Dabrowski for box beams with continuous diaphragm medium and Resinger and Richmond for box beams with concentrated diaphragms. A comparative investigation of these methods and Benscoter’s theory for box beams with rigid cross sections have been carried out. Dabrowski extended his work to the analysis of box beams curved in plan. He produced a general theory for circular box beams with rigid cross sections and derived solutions for several loading systems. The writer has considered a different support condition for which similar solutions have been derived. Dabrowski suggested an approximate method of analysis when distortion of the cross section is taken into account, and produced some series solutions for the governing differential equation for single span beams. The writer has produced a more detailed analysis of single span beams and has derived exact solutions for loads at the supports which were used to extend Dabrowski methods to continuous beams. Two model tests were carried out, both using perspex models. The first was on a straight beam, 66 in. long and 6 in. square, with a constant wall thickness of in. The second was on a two span continuous circular box beam, each span of 84 in. and a radius of 140 in., the cross section was 8.025 in. square with a varying thickness of 5/32 in. at the top wall and 1/8 in. at the other walls. Both models were tested under vertically acting distributed twisting loads of uniform intensity. The stiff-Jointed rectangular frame formed by the walls was assumed to provide a continuous diaphragm medium. The comparison of the theoretical and experimental deflexions for the first test showed a good agreement between them. For the second test, reasonable agreement was obtained, and it is probable that the discrepancies were mainly, due to experimental errors.